Quality assurance standards particularly in critical areas of aerospace construction such as aircraft and space vehicles have become increasingly stringent as the need to improve the predictable life and performance of such structures has increased. Holes formed in metal structures to receive fastener devices such as rivets, bolts or special fasteners are subject, particularly under the pressures of mass production, to the presence of flaws such as burrs, scratches, gouges, out-of-roundness, and others. Holes may also be formed at or near the presence of slight imperfections in the metal which can affect hole quality. If undetected, such flaws can seriously impair the mission life of a given structure by resulting in premature failure due to fatigue or other cracking at the fastener hole.
One previous method of inspection for flaws in or at fastener holes has been the use of eddy current detection. In this method a small energized electrical coil is passed over the interior surface of the hole thereby inducing eddy currents in the surrounding metal. Presence of flaws or aberrations is ascertained by detecting changes in current flow in the coil circuit resulting from its altered effective inductance when the character of the metal structure changes, for example, if a cut or gouge is present. Conventionally, the coil is incorporated into the tip of a small rod-like probe which is passed into the hole manually or by mechanical means. Manual use, however, is slow and subject to erroneous readings due to uneven and unsteady insertion. In the prevalent previous mechanized method the probe is inserted by mechanical means directly into the hole where it is held by spring action against the hole inner surface and moved only in a predetermined helical pattern, e.g., by constant linear advance with a constant rate of rotation. In these uses, however, it has been found that scratches and other flaws in the hole inner surface can scar and abrade the material of the probe which is typically a resinous plastic material. When this occurs, then even over a short period of repeated use the probe tip may be so worn as to cause shorting out of the probe coil thus destroying usefulness of the probe and requiring its replacement. Moreover, with conventional eddy current probes detection sensitivity is highly affected by lift-off frequency settings. "Lift-off" frequency is that electrical frequency to which the probe coil circuit is adjusted to minimize the "noise" which occurs when the probe just separates or lifts off the surface of the material being tested. In prior art use of a probe inserted directly into the hole, the probe tip may at times insufficiently contact the hole inner surface. This results in undesirable lift-off "noise" signals which tend to obscure the electrical signatures of flaws. While normally the circuit driving frequency is tuned for minimal reaction to surface lift-off, in practice the circuit often becomes slightly detuned with resulting increase in noise leading to signal misinterpretation or missed flaws and the necessity of repeated tuning. Non-axially aligned probe insertion may also contribute to noise and spurious signals.
Other drawbacks to prior eddy current inspection of holes include its being generally insensitive to detecting out-of-round holes and the heretofore limited mechanized modes of probe movement which are inadequate for the precision computer control of the inspection process contemplated herein. As a result of these factors, use of eddy current inspection has been restricted to less than its potential.